We report two females, and one male with Isaacs' syndrome. The patients presented with clinical myokymia activity, muscle cramps, delayed relaxation, and muscle hypertrophy and increased sweating. Needle electromyography in several muscles showed generalized continuous motor unit discharges, myokymic discharges, and normal nerve conduction studies. Muscle biopsy showed type two fiber atrophy. Treatment with carbamazepine was effective in two cases and prednisone in one.